How to mend a sash window

There are few things more frustrating than a window that won't open, because whatever the reason for the problem - whether it's mechanically compromised, or just jammed because subsidence has caused your house to lean 12 degrees to the right - the solution usually seems totally beyond one's control. Even a window that's been painted shut can be maddeningly difficult to shift. I've shattered a few trying.

For this reason, and others, I put off fixing the old-fashioned sash window in my kids' bedroom for many years. In their younger days I was more concerned that they would leap out of it, and dressed up my laziness as a precaution. In the hottest days of last summer I was finally able to force it partway up and prop it open, but it was so heavy that I was afraid it would snap the section of curved toy train track I'd used to wedge it in place. I was even more afraid that one of my kids would one day find himself short of curved track and try to pull it out, guillotining his hand off in the process. It needed fixing.

I was, though, more than a little daunted by the idea of repairing a sash window myself, for while I know how they work in principle - a fairly primitive arrangement of weights, cords and pulleys - I've never actually seen the mechanism exposed to the air. Believing that I would be able to fix it seemed, at this time of year, doubly foolhardy: realistically, it would be months before I really needed to open the window, but if I were to screw things up, I could easily spend the winter with a big square hole in my house ...

How to...

1. Locate offending part

Well, at least I know the problem (presuming, of course, there is only one problem). The right-hand sash cord is broken. In theory, all I need do is buy some more cord, attach it to the lower sash (a general term for a moveable window panel) at one end and to the weight at the other, and everything should work fine. In practice, I don't even know how to get the window off.

2. Remove lower window

Fortunately, this proves simpler than I'd imagined. The lower sash is prevented from falling into the room by two thin, vertical strips of wood, which I now know to be called "inner staff beads". Though they look like part of the window moulding, they are in fact detachable and held in place by half a dozen small, headless nails. Inserting a chisel between the bead and the moulding and tapping it with a wooden mallet is enough to pry it away, albeit with an alarming cracking of old paint.

Once the beads have been removed (whole or in pieces), the lower sash should come out easily, unless it's still attached to one of the sash cords. If so, just cut it (you might as well replace both cords while you're here) - taking care not to let the attached sash weight smash down to the bottom of its cavity. Put the sash somewhere out of the way - so you don't put your foot through one of the panes while you're working ...

Note: Behind the lower sash, you'll discover that there exists a vertical "parting bead", which separates it from the upper sash. You only need to remove this if the top sash is also in need of repair. If you want both your upper and lower sash to move freely you are, in my opinion, a person of unseemly ambition.

3. Remove sash weights

In either side of the window frame where the lower sash travels, you'll find the compartments where the weights are hidden. These are generally accessible via a panel of wood, technically known as a "pocket", which will hopefully be sitting snugly in place and can be pried out - unless, of course, a previous occupant of your home decided to screw or nail it in place in order to make things difficult for you decades down the line. Behind each pocket you will find the iron sash weight, longer and heavier than you imagined. You may be the first person to gaze upon these in a hundred years. Take them out and admire.

Now is also a good time to make sure both pulleys are operating properly, because they probably aren't. Mine were both gummed up with layers of old paint, and it took a bit of scraping to get them turning freely. A drop of oil wouldn't hurt, either.

4. Fix new cords to the weights

So this is what you should have before you: two parting beads (one snapped in half), removed; one lower sash, removed; two sash weights, removed; one window, minus lower sash, with two extra long lengths of new sash cord running up and over the pulley, down into the sash weight compartment and out again at the bottom. Now it's time to tie the new cords to the old sash weights. This is achieved by feeding the cord through the hole in the top of the weight and out of the side, and then tying a figure-of-eight knot (or stop-knot) in the end so it won't pull back through.

Revelation! It was at this point I discovered that sash cord comes in different widths, and that I'd bought the wrong sort, even though I took a piece of the old sash cord with me to the DIY superstore. The new stuff was too skinny and the knot pulled through the weight's hole with the slightest tug. What was required was a fatter "double-eight" knot - I copied an example from a little framed sampler of nautical knots which I found in my son's bedroom.

5. Replace the lower sash

Once the cord is finally secure, cut off any excess, take up the slack from the top, reinsert your sash weights and tap the little wooden panels (pockets) back into place. The next thing to contemplate is the manner in which the sash cords are affixed to the sash. Sometimes a stop-knot arrangement is used, but more often, I think, the cord is tacked into a groove with three or four "clout" nails. Or just re-use the nails you pulled out to remove the old bits of sash cord, and don't worry about what they're called.

6. The end game

(For this step you will probably require an extra pair of hands.)

Put the sash back in place, but leave it leaning out a little at the top. Then pull the sash weights right up to the pulley, keeping the cord as taut as possible as you nail it into the groove on the side of the sash frame (as explained above). Remove any excess cord with a sharp knife. Once both cords are secured, the window should sit perfectly in place and slide up and down easily. Now take a step back. Looks a mess, doesn't it? You'll need to paint it, of course ...

Tip no 1

You should start in the middle of the bead so you don't snap it in half - although, come to think of it, it's perfectly possible to start in the middle and snap it in half anyway. I did.

Tip no 2

If there's a bit of the broken sash cord attached to the weight and still hanging out of the pulley at the top, it might be a good idea to tie a length of string to it, so that when you remove the weight, you can use the string to pull the new cord into place (alternatively, tie some string to a nail and feed it through from the top, allowing the weight of the nail to pull it, and then the attached cord, over the pulley and down to sill level). Leave the new sash cords long and trailing out at either end.

Tip no 3

When, finally, you go to nail the inner staff beads back into place, tack them in gently first to make sure they aren't interfering with the free movement of the sash. They might need a bit of adjusting.

Sign up for the Guardian Today

Our editors' picks for the day's top news and commentary delivered to your inbox each morning.